For the general philosophically-inclined reader this book should be an enjoyable immersion in the intellectual life of the 17th/18th centuries, seen through the intertwined lives and thought of Spinoza and Leibniz. Stuart brings alive the European world of intellectual foment of the time – galvanised by emerging rationalism, freethought and science – through a mix of engaging everyday detail and serious philosophical investigation. The differences in temperament of the two philosophers are highlighted, perhaps overstated even, in order to create an engaging drama of the mind.

Thinkers of this period, and particularly the three so-called ‘rationalists’ – Descartes, Spinoza and Leibniz – hold an enduring fascination for us, but, for many contemporary writers, they become a blank slate onto which contemporary issues are projected. For example, Gilles Deleuze wrote monographs on Spinoza and Leibniz which, for me at least, paint quite unrecognisable portraits of them, while Antonio Damasio has used both Descartes and Spinoza as starting-points for promoting the latest findings in neuroscience, and which miss out everything essential in the thought of those two men. Stuart largely avoids such traps, by foregrounding a simple fact of the lives of Spinoza and Leibniz: that their being and thought were framed by a profound engagement with questions of religion. For example, Stuart says of Spinoza that he ‘rejected the orthodoxy of his day not because he believed less, but because he believed more.’ I think that this could stand for many great Enlightenment thinkers, though their contemporaries often called them ‘atheists’ for it, and which label has persisted into modern secular times.

But what is Stuart’s original thesis that drives his account of the lives and philosophies of Spinoza and Leibniz (otherwise so well covered by other authors)? Firstly, in his contrasting of the temperaments of the two men: Spinoza as modest, self-sufficient and in repose, and Leibniz as boastful, seeking praise, and in ever-shifting dependency on those whose favour he courted. Secondly, that Leibniz was deeply influenced by Spinoza. Thirdly, that Leibniz denied that influence, both to himself and the outside world, leading to a ‘return of the repressed.’ And fourthly, that Spinoza was a modernist, embracing the emerging materialism that would define the secular world to come, while Leibniz was an anti-modernist, clinging to baroque modes of thought.

Now at this point I have to confess that I am not the best reviewer to unpick some of the finer points of philosophy per se. Goethe was reputed to have said that he did not have the ‘proper organ for philosophy,’ and I think I am likewise handicapped. Although I have studied philosophy extensively, in the end I often find myself indifferent to its niceties. For example, in Stuart’s juxtaposition of the philosophies of Spinoza and Leibniz, much is made of their different positions on ‘substance’ – whether the world is one substance (God) differently modified, or whether it is a plurality of substances – the keenness of which argument derived from the Europe-wide debate over Descartes’ distinction between res cogitans and res extensans. Stuart’s job, as a dramatist of philosophical history, is to make this difference an almost life-or-death issue (which to some extent it was at the time), but which for me is ultimately a futile argument: I don’t see how one can live one’s life from a definition of existence reduced to such impoverished terms. But there is more to my anxiety about Stuart’s thesis than this: it is that to some extent I identify with Leibniz when he is characterised as a polymath irenicist, or as Stuart puts it, a man who wanted to be the ‘Great Peacemaker of All Thought.’ Of course, Stuart’s scholarship on Leibniz is infinitely beyond my own, and it may be that his portrait of Leibniz is accurate. But I find that too often he takes a known fact about him, for example that he was not a well-built man, and turns it into a prejudicial image, for example when leading up to the dramatic and crucial meeting between the two men. Stuart describes Leibniz that he stepped ashore: ‘arms flapping, wig billowing, perfume dissipating in the autumn wind, and gambolled in his awkward way along the leaf-strewn canals …’ The image may serve Stuart’s literary purpose, but is it fair to Leibniz?

I have long been interested in the three ‘rationalists’ despite my ultimate indifference as to their formal contributions to philosophy: it is their religious thought that draws me, as I find in it a location within the traditions of Western mysticism. More than that: by exploring that context, some of Stuart’s deductions about their philosophy become questionable. In particular the conclusion that Spinoza is a modernist (an assumption also made in the favourable treatment of Spinoza by Damasio) is cast into doubt. But it is Leibniz’s use of the term ‘monad’ that gives the clue: contrary to Stuart’s assertion that this term was first used by Giordano Bruno, it is found in the mystical work known as The Divine Names written by Dionysius the pseudo-Areopagite of the third century. There is a clear trajectory of European mystical thought from Dionysius, through Erigena and Eckhart, to the Renaissance neoplatonism of Ficino; at the head of which stands Plotinus, and behind him much Greek thought including Pythagoras. In different ways, Descartes, Spinoza and Leibniz are inheritors of this tradition, one characteristic of which is an extreme denial of the personhood of ‘God’ – an anti-anthropomorphism that Stuart mistakenly believes to be a sign of modernism. When Spinoza, as a Jew, is understood to have also inherited the profound anti-anthropomorphism of the Jewish mystical tradition (as described so well in Gershom Scholem’s classic work on the subject), it cannot be so clearly argued that his stance was modern over Leibniz as regressive. Better, perhaps is Stuart’s central idea: that the different temperaments of these two men lead to their differing but intertwined philosophies. Because, Leibniz, even more than Spinoza, had studied the Western esoteric and mystical traditions, including the Kabbalah. And it is in his Monadology that we can find an extraordinary and original recapitulation of Western mystical thought going all the way back to Pythagoras: the central idea of which is ‘man as microcosm.’

If one has the ‘organ for philosophy’ then Stuart’s conclusion, that Spinoza’s ‘substance monism’ is a form of secular reductionist materialism, and that Leibniz opposed this, may be correct. If, however, on has the ‘organ for mysticism’ then one cannot agree that Spinoza’s ideas represent a materialism of any kind, rather that his pantheism makes the whole world sacred, even in the most mundane of places and experiences. And one would find that Leibniz’s Monadology does the same and more: for example, it anticipates Searle’s Chinese Room Paradox, and the findings of quantum theory, namely quantum entanglement. These gems of the Monadology don’t mean that Leibniz is ‘modern’ either: simply that his originality of thought was at least that of Spinoza’s, and, mostly, independent of it.

But these caveats should not detract from Stuart’s work: his book certainly had me deeply absorbed, and provoked interesting trains of thought. (One point in particular pleased me: his acknowledgement that Kant’s categorisation of Descartes, Spinoza and Leibniz as ‘rationalists,’ and Berkeley, Locke and Hume as ‘empiricists,’ is absurd.) The richness of research and imaginative reconstruction in Stuart’s book will elicit different responses from different readers, but for me it leaves me with one final thought: can I reconcile what I find in the Monadology as an expression of the highest mystical insight (on a level with that of Plotinus, Eckhart and the Buddha) with a portrait of a man so deeply flawed?